This proposal is concerned with the personality categories that people use to encode those individual differences which they encounter in everyday life. We develop a semantic model of personality traits as hierarchically organized categories whose instances are behavioral acts of varying degrees of prototypicality. First, we establish a pool of traits varying in category breadth; employing a new measure of category breadth, we will show that broad traits refer to more diverse acts and are more meaningful than narrow ones, and we will demonstrate asymetrical implications for trait pairs varying in category breadth. Second, in studies of the prototypicality of behavioral acts, we extended our model by specifying situational settings, analyzing trait confirmatory and disconfirmatory acts, and accounting for multiple categorizations of behaviors. Third, various experiments are proposed in order to determine whether there is a level of categorization that is particularly informative or helpful, and how it changes as a function of the target information available to the perceiver, the purpose of the task, the current state of the perceiver, and stable perceiver characteristics. A final set of studies tests some implications of our hierarchical model of trait categories for issues in personality assessment.